1. Field of the Invention
The subject invention pertains to a method for detection of atrial fibrillation, and an apparatus suited therefor.
2. Description of the Related Art
Acquired illnesses of the heart can be divided into illnesses of the endocardium, of the myocardium, of the pericardium, and of the conduction system.
Systematic recording and consideration of the classical risk factors such as excess weight, high blood pressure or a high cholesterol level allows no more than barely 60% of the patients at risk to be identified in good time.
Invasive clinical examination methods, such as the cardiac catheter method, which is carried out 500,000 times a year in Germany alone and is associated with considerable risks, allow a reliable prediction of whether the patient examined is under threat of a heart attack at present or any other illness of the heart. One patient in a thousand dies with this method of diagnosis, however.
Non-invasive clinical methods of examination, such as electrocardiography, are not associated with such great safety risks. However, the clinical standing and reliability of this diagnostic are likewise unsatisfactory on the basis of the prior art and is too greatly dependent on the frequency with which the individual symptoms occur over time.
Electronic imaging methods, such as magnetic resonance imaging, will replace the invasive cardiac catheter method over the course of time. Electronic imaging methods allow inflammatory foci in vessels to be easily identified, and in principle to also be easily diagnosed as a result of their different tissues. However, this equipment represents a high capital investment and is also very cost-intensive in diagnostic use. Moreover, it is ill-suited to long-term observation, since patients generally cannot tolerate the relatively long time spent in the narrow tubes for accommodating the body.
Biochemical methods are generally based on blood examinations using “biomarkers”. The best known method involves measuring the protein CRP, which indicates inflammatory processes in the body. This “CRP test” gives an indication allowing a patient's health risk to be estimated at least more precisely.
One good way of detecting various cardiac damage early on an electronic basis is provided by the actual heart's electrophysiological conduction system. The conduction system manifests itself differently in the individual tissue types through different electrical potential patterns, produced electrophysically by electrical polarization and depolarization.
Besides the ventricular and superventricular extrasystoles, atrial fibrillation is the most frequently occurring arrhythmia. Assuming a mean prevalence of atrial fibrillation between 0.4% and 1% of the population, there are between 330,000 and 830,000 citizens affected in Germany alone. Taking into account the fact that atrial fibrillation also increases with age and that the proportion of older people in the total population is increasing, the number of people affected will naturally also rise continually.
Atrial fibrillation is characterized by electrical excitation waves which propagate without synchronization in the atrial myocardium and result in chaotic depolarization sequences with hemodynamically ineffective atrial constrictions.
These biophysical conformities to basic laws were the starting point for development of the clinical non-invasive electrophysical diagnosis method of electrocardiography, i.e. of curve-based recording of the electrical excitation waves in the “electrocardiograms” (ECG). From the ECG it is possible to infer the heart rhythm, the heart rate, the eptopic beat and the conduction, in principle using different methods and with varying exactness. A distinction is drawn between ECG at rest, ECG under stress and long-term ECG.
In the case of atrial fibrillation, the ECG at rest shows “QRS complexes” arranged at irregular times (this is referred to as absolute arrhythmia), while the baseline has irregular fibrillation waves (the “f waves”) of different amplitude and shape. Depending on the arrangement of the electrodes on the patient's body, the fibrillation waves cannot be identified with sufficient safety in all measurements, which means that the diagnosis “atrial fibrillation” can be ascertained to a sufficient extent only from the temporally irregular successions of the QRS complexes. With very fast and very slow ventricular transmission, however, the ventricular rate can appear relatively regular, giving rise to the possibility of an incorrect diagnosis of “absolute arrhythmia”.
It is therefore necessary for the RR intervals for the QRS complexes to be evaluated very precisely over a relatively long period of time in order to keep the measurement deviations for rate determination in the ECG at rest within permissible limits.
The ECG under stress is a test method for estimating the heart rate under rest and stress conditions. It can thus be used to estimate the biological effectiveness of antiarrhythmics. A reduction in the heart rate does not always signify an improvement in the heart's work, however. The ECG under stress thus cannot sufficiently detect the functional cardiopulmonary stages of a patient with atrial fibrillation.
The long-term ECG is a measurement method for detecting and recording proximally occurring atrial fibrillation. There is thus the opportunity to detect both spontaneously occurring intermittent disturbances in rhythm and “trigger arrhythmia”. Normal measurement times are approximately 24 to 72 hours. Long-term electrocardiography is probably the most important method of detecting and hence of diagnosing atrial fibrillation at present. Its value is in the diagnosis of symptoms which occur at least once a week. Events which occur less frequently cannot be detected using this measurement method, however. Since the unit is relatively heavy, its use on a mobile basis is limited, which means that the measurement time cannot be increased to the extent required medically.
This problem can be significantly alleviated using the “event recorder”. This is an ECG recorder which, like the long-term ECG, is fixed to the patient using electrodes but has a much lower weight and physical volume. However, the recorder, weighing only a few grams, has a markedly limited storage capacity which permits ECG recording over just three minutes. The result of this is that the patient needs to press an event button when a clinical event occurs in order to start the ECG storage. Clinical experiences using the event recorder to detect atrial fibrillation are therefore also very limited. Following a recent study, a correct ECG diagnosis was able to be made in only 68% of a statistical collective with symptoms such as “palpitations”, for example.
In summary, it can be said that the measurement and hence also the diagnostic certainty is too greatly dependent upon the frequency of occurrence of the corresponding symptoms. Hence, the clinical standing of this non-invasive diagnostic is not very high.
WO 02/24086 A1 discloses a system for detecting atrial fibrillation, including an evaluation circuit which calculates the distribution of RR intervals from the patient's heart potentials and compares it with at least one known distribution to generate a state signal representing the state of the patient's heart. This system involves measuring the RR intervals, forming a histogram of the ΔRR deviations, and comparing the histogram with histograms of other patients suffering from arrhythmia.